The present invention generally relates to amplifier circuits, and more particularly to an amplifier circuit having a feedback circuit and a high-gain amplifier.
A circuit having a high-gain amplifier, such as a limiter amplifier, amplifies a noise component when no input signal is applied to the high-gain amplifier. Thus, it is necessary to prevent the circuit having the high-gain amplifier from outputting amplified noise components when no input signal is applied thereto.
FIG.1 shows a conventional amplifier circuit having a high-gain amplifier. The amplifier circuit shown in FIG.1 has a high-gain amplifier (AMP) 10, such as a limiter amplifier. A filter 11, which is provided on the input side of the high-gain amplifier 10, functions to prevent noise components from being input to the high-gain amplifier 10. The filter 11 has a narrow frequency band designed to have frequencies other than an inherent oscillation frequency of the high-gain amplifier 10. In place of or in addition to the filter 11, a shield case 12 is provided so that it shields the high-gain amplifier 10. The shield case 12 prevents the high-gain amplifier 10 from being affected by noise.
However, the use of the filter 11 or the shield case 12 cannot completely eliminate the influence of noise on the amplifier. Particularly, it is very difficult to completely prevent the outputting of noise signals by means of the filter 11 or the shield case 12.